A substance used to reduce the viscosity of paints or coatings.

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Reducing the thickness of a coating so it can flow smoothly and apply evenly is done with a thinning agent. A thinner or solvent lowers the viscosity of the paint or coating, improving sprayability, wetting, and leveling, and it generally evaporates as the coating cures, leaving little to no residue. The other options have different primary roles: a hardener cures the coating chemically, a binder is the film-forming component that holds the coating together, and additives are miscellaneous agents that modify specific properties but don’t primarily thin the material.